The present invention relates generally to optical network, and more particularly to optical mirror coatings for high-temperature diffusion barriers and mirror shaping.
The following references are used through this patent application, and are hereby incorporated herein by reference in their entireties:
[1] K. Nunan et. al., LPCVD and PECVD Operations Designed for iMEMS Sensor Devices, Vacuum Technology and Coating, January 2001, pp. 27-37;
[2] R. F. Bunshah et. al., Deposition Technologies for Films and Coatings;
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Mirrors used in optical applications, such as optical networking applications, must be highly reflective in order to minimize dispersion and reduce optical signal loss. These optical mirrors often consist of a thin film of highly-reflective material, such as gold, platinum, or aluminum, layered directly or indirectly over a substrate, such as single crystal silicon or polysilicon. The mirror layer is typically very thin in order to reduce problems from film stress and thermal expansion. The mirror layer may be placed directly on the substrate, although the mirror layer is quite often separated from the substrate by one or more additional material layers. For convenience, the material onto which the mirror layer is placed (whether the substrate or an additional material layer) is referred to hereinafter as the xe2x80x9cbacking layerxe2x80x9d for the mirror layer.
One reason for using a backing layer is to improve adhesion of the mirror layer onto a lower material layer (whether the substrate or an additional material layer). Depending on the types of materials used in the optical mirror, the mirror layer may not adhere well to the lower material layer if placed directly on top of the lower material layer. Therefore, a backing layer may be used to bond the mirror layer to the lower material layer. In order to effectively bond the mirror layer to the lower material layer, the backing layer material must adhere well to both the lower material layer and the mirror layer.
Another reason for using a backing layer is as a diffusion barrier to prevent interdiffusion and/or the formation of intermetallics between the mirror layer and a lower material layer (whether the substrate or another material layer).
An intermetallic is a type of alloy containing two or more metal atoms. In studies of gold films, it has been found that gold and silicon react at temperatures considerably below the eutectic (363C) [3], that gold thin films react with metals at room temperature if both the metal and the resulting compound(s) have melting points below approximately 700C [4], and that the reaction rate of intermetallic formation is controlled by diffusion and the interdiffusion coefficient is linearly dependent on melting point [5]. This linkage between intermetallic formation and diffusion was also noted in a study of gold-titanium, where diffusion was measurable at temperatures above 175C, allowing formation of intermetallics at temperatures above 250C [6].
Interdiffusion is essentially the mixing of two materials due to random thermal motion. Interdiffusion can occur, for example, when atoms from the lower material layer diffuse into the mirror layer or when atoms from the mirror layer diffuse into the lower material layer. In order for interdiffusion to occur, an atom must acquire a sufficient amount of energy to leave its present material so as to become available for diffusion into the other material, and there must be a xe2x80x9cfree volumexe2x80x9d in the other material into which the atom can diffuse. Heat can provide the energy required by the atom to leave its present material and/or the energy required (if any) to create xe2x80x9cfree volumexe2x80x9d. As-deposited metal films have more lattice defects (vacancies, grain boundaries, and dislocations) than do annealed films that are in thermal equilibrium. It has been found that grain boundary diffusion in gold is at least an order of magnitude higher than bulk diffusion [3]. Thus, small grain size films may be more susceptible to diffusion than coarser films. One way to avoid grain boundary diffusion is to use an amorphous thin film as a diffusion barrier, since amorphous films have no grain boundaries. For example, it has been found that amorphous nickel-niobium [7], amorphous tungsten-nitride [8], and amorphous tantalum-silicon-nitride [9] do not readily interdiffuse with silicon or gold.
Depending on the types of materials used in the optical mirror, interdiffusion and/or the formation of intermetallics may occur between the mirror layer and the lower material layer if the mirror layer is placed directly in contact with the lower material layer. Interdiffusion and/or the formation of intermetallics may contaminate the mirror and reduce the mirror""s reflectivity. Therefore, a backing layer may be used to prevent interdiffusion and/or the formation of intermetallics between the mirror layer and the lower material layer. In order to effectively prevent interdiffusion and/or the formation of intermetallics between the mirror layer and the lower material layer, the backing layer material must not interdiffuse or form intermetallics with the mirror layer and must prevent the lower material layer from interdiffusing and forming intermetallics with the mirror layer.
Another reason for using a backing layer is to physically strengthen the mirror layer to prevent undue stresses in the mirror layer. Stresses in the mirror layer can cause gradual changes in the microstructure of the mirror layer material [10]. In a study focusing on a nickel-iron alloy for magnetoresistive elements [11], an approximate relationship was found between the melting temperature of a metal and the onset of intermetallic formation, and a link was also found between the onset temperature and diffusion. In a study focusing on diffusion and intermetallic formation in gold-niobium electrical contacts [12], interdiffusion, intermetallics, and gold hillocks (which were objectionable because they interfered with electrical contacts) were observed at temperatures above 350C.
A common optical mirror configuration uses a thin gold film as the mirror layer and uses silicon as the substrate. The gold mirror layer can be placed directly on the silicon substrate. However, the gold mirror layer does not adhere well to the silicon substrate, and therefore the gold mirror layer can be easily damaged. Also, the silicon and the gold tend to interdiffuse and form intermetallic compounds, even at relatively low temperatures. This contaminates the gold mirror layer, which can reduce the mirror""s reflectivity.
Therefore, a backing layer of chromium or titanium is often used between the gold mirror layer and the silicon substrate in order to improve adhesion and prevent contamination. The chromium or titanium backing layer adheres well to the silicon substrate, and the gold mirror layer adheres well to the chromium or titanium backing layer. The chromium or titanium backing layer does not interdiffuse or form intermetallics with the gold mirror layer at relatively low temperatures, particularly at temperatures at which the optical mirror is typically used. The chromium or titanium backing layer also acts as a diffusion barrier to prevent the silicon substrate from interdiffusing with the gold mirror layer.
Even though the optical mirror is typically used at relatively low temperatures, the optical mirror may be subjected at times to high temperatures. For example, the optical mirror may be exposed to several high temperature processes when the optical mirror is assembled into a hermetically-sealed package. At these high temperatures, the chromium or titanium backing layer can interdiffuse with the gold mirror layer and contaminate the gold mirror layer. Extensive interdiffusion of gold and chromium has been observed with exposure to a temperature of 300C for ten minutes, and many typical assembly processes use temperatures near and above 300C. Thus, even though a high-quality gold-over-silicon optical mirror can be constructed by using a chromium or titanium backing layer, the gold-over-silicon optical mirror can be destroyed through various high-temperature ancillary processes. Similar problems can occur in other optical mirror configurations from exposure to high temperatures.
Certain materials for use as high-temperature diffusion barriers under optical mirror coatings include metals that have high melting and/or boiling points and amorphous and partially recrystallized inorganic amorphous materials that have high glass transition temperatures (Tg). Candidate metals are selected based upon the boiling point or a combination of melting point and boiling point. Candidate amorphous and partially recrystallized inorganic amorphous materials are selected based upon the glass transition temperature. Optical mirrors having such high-temperature diffusion barriers maintain reflectivity when exposed to elevated temperatures, and are particularly useful in optical Micro Electro-Mechanical Systems (MEMS) that are exposed to high-temperature manufacturing processes.
In accordance with one aspect of the invention, an optical mirror includes an optical mirror coating and a high-temperature diffusion barrier that does not readily interdiffuse with the optical mirror coating at selected elevated temperatures.
The high-temperature diffusion barrier may consist of any of a variety of high-temperature metals. The high-temperature diffusion barrier may be a high-temperature metal having an atmospheric boiling point (BP) above approximately 3550 degrees Kelvin. The high-temperature diffusion barrier may be a high-temperature metal having a melting point and an atmospheric boiling point such that the product of the melting point and the atmospheric boiling point (MP*BP) is above approximately 7.5xc3x97106K2. Exemplary high-temperature metallic diffusion barrier materials include vanadium, platinum, rhodium, zirconium, hafnium, iridium, ruthenium, niobium, molybdenum, osmium, tantalum, rhenium, and tungsten.
The high-temperature diffusion barrier may consist of any of a variety of high-temperature amorphous materials, and, in particular, any of a variety of high-temperature amorphous solids having a glass transition temperature above approximately 500 degrees Celsius. Exemplary high-temperature amorphous diffusion barrier materials include amorphous titanium nitride, amorphous nickel-niobium, amorphous tantalum-silicon-nitride, amorphous tungsten nitride, amorphous silicon nitride, and amorphous low-stress silicon nitride.
The high-temperature diffusion barrier may consist of any of a variety of partially recrystallized inorganic amorphous materials having a glass transition temperature above approximately 500 degrees Celsius. Exemplary high-temperature partially recrystallized inorganic amorphous materials include an amorphous tungsten nitride material having tungsten nitride crystals dispersed with tungsten crystals.
In accordance with another aspect of the invention, an apparatus, such as an optical Micro Electro-Mechanical System (MEMS), includes a optical mirror having an optical mirror coating and a high-temperature diffusion barrier that does not readily interdiffuse with the optical mirror coating at selected elevated temperatures.
In accordance with yet another aspect of the invention, a method for producing an optical mirror involves selecting a high-temperature diffusion barrier material and depositing the high-temperature diffusion barrier material underneath an optical mirror coating. Selecting the high-temperature diffusion barrier material may involve selecting a high-temperature metallic material based upon an atmospheric boiling point of the material or a combination of the melting point and the atmospheric boiling point of the material. Selecting the high temperature diffusion barrier material may involve selecting a high-temperature amorphous or partially recrystallized inorganic amorphous material based upon a glass transition temperature of the material.
In accordance with yet another aspect of the invention, a method for forming a substantially non-flat optical mirror involves forming at least one film layer on at least one of a front side and a back side of a mirror stack layer such that the film stresses of the at least one film layer shape the optical mirror into a predetermined substantially non-flat shape. The at least one film layer may include a tensile film and/or a compressive film. In one embodiment, the predetermined substantially non-flat shape is a substantially concave shape, which can be formed by forming a tensile film on the front side and/or forming a compressive film on the back side. In another embodiment, the predetermined substantially non-flat shape is a substantially convex shape, which can be formed by forming a tensile film on the back side and/or forming a compressive film on the front side.
In accordance with yet another aspect of the invention, a substantially non-flat optical mirror includes at least one mirror stack layer having a front side and a back side and at least one film layer formed on at least one of the front side and the back side such that the film stresses of the at least one film layer shape the optical mirror into a predetermined substantially non-flat shape. The at least one film layer may include a tensile film and/or a compressive film. The substantially non-flat optical mirror may have a substantially concave shape, and may have a tensile film formed on the front side and/or a compressive film formed on the back side. The substantially non-flat optical mirror may have a substantially convex shape, and may have a tensile film formed on the back side and/or a compressive film formed on the front side. The optical mirror may be a micro machined optical mirror, and the mirror stack layer may be a single crystal silicon or polysilicon layer.
In accordance with yet another aspect of the invention, a method for forming a substantially flat optical mirror involves forming at least one film layer on one of the front side and the back side of a mirror stack layer such that the film stresses of the at least one film layer shape the optical mirror into a predetermined substantially flat shape. The substantially flat optical mirror can be formed using tensile and/or compressive films.
In accordance with yet another aspect of the invention, a substantially flat optical mirror includes at least one mirror stack layer having a front side and a back side and at least one film layer formed on one of the front side and the back side such that the film stresses of the at least one film layer shape the optical mirror into a predetermined substantially flat shape. The substantially flat optical mirror can be formed using tensile and/or compressive films. The optical mirror may be a micro machined optical mirror, and the mirror stack layer may be a single crystal silicon or polysilicon layer.
In accordance with yet another aspect of the invention, a method for producing an optical mirror having a predetermined target shape involves forming an optical mirror stack having a plurality of mirror stack layers, where each mirror stack layer has an initial film stress, and the combined initial film stresses of the plurality of mirror stack layers shapes the optical mirror stack into an initial shape different from the predetermined target shape. The film stress of at least one mirror stack layer is subsequently altered so as to change the shape of the optical mirror stack from the initial shape to the predetermined target shape.